The present invention relates to a wheel-support rolling bearing unit, which is used for rotatably supporting vehicle wheels on suspension devices, and also a method for manufacturing the same.
The vehicle wheels are supported on the suspension devices by the wheel-support rolling bearing unit. A wheel-support rolling bearing unit is known which structured not using nuts for firmly coupling the hub and the inner ring for the purposes of cost reduction by reducing, the number of the number of component parts, and the size and weight reduction, as disclosed in Japanese Patent Unexamined Publication No. Hei.11-129703. FIGS. 4 through 6 show the wheel-support rolling bearing unit 1 disclosed in the publication.
The conventional wheel-support rolling bearing unit 1 includes a hub 2, an inner ring 3, an outer ring 4, and a plurality of rolling elements 5. A first flange 6 for supporting the wheel is formed at a portion of the hub 2 which is located closer to the outer end of its outer peripheral surface (the word “outer side or outside” means the outer side of the vehicle body as viewed in the widthwise direction in a state that the unit is assembled into the motor vehicle, and the left side in those figures except FIGS. 3 and 7. The word “inner side or inside” means locations closer to the center of the vehicle body as viewed in the widthwise direction, and the right side in those figures except FIGS. 3 and 7.). A first inner raceway 7 is formed in the outer surface of a middle portion of the hub 2, and a stepped part 8 whose outside diameter is small is formed at the inner end thereof. The inner ring 3 is fit to the stepped part 8 and fastened by a caulking portion 9. The first inner raceway 7 is formed directly in the outer surface of the middle portion of the hub 2, and sometimes is formed in the outer surface of a separate inner ring fit to the middle portion of the hub. In the later case, a portion of the end of the hub 2, which is protruded to the inner side as viewed in the axial direction beyond the separate inner ring, serves as the stepped part which receives the inner ring 3.
To this end, a cylindrical portion 10, which is used for forming the caulking portion 9, is formed in the inner end of the hub 2. A thickness of the cylindrical portion 10 becomes small toward its top end before the cylindrical portion 10 shown in FIG. 6 is expanded, for caulking, outward in the diameter directions. Accordingly, a tapered hole 11 is formed in the inner end face of the hub 2. The inside diameter of the tapered hole becomes small toward a recess.
To expand, for caulking, the top end of the cylindrical portion 10 in order to fasten the inner ring 3 to the inner end of the hub 2, a force piston 12 is forcibly pressed against the top end of the cylindrical portion 10 in a state that the hub 2 is fixed so as not to shift in the axial direction, as shown in FIG. 5. An outward curved portion 13, shaped like a truncated cone, which may be thrust into the cylindrical portion 10, is formed in the central portion of the end surface (left end surface in FIG. 5) of the force piston 12. An inward curved portion or recess 14 is formed surrounding the outward curved portion 13. A cross section of the recess 14 is configured to have a complex curved surface whose radius of curvature becomes small toward the outside so that a cross section of the caulking portion 9, which is formed by plastically forming the top end of the cylindrical portion 10 by the recess 14, gradually reduces in size from the base end to the top end, and abruptly reduces at the top end.
By pressing the force piston 12 having the outward curved portion 13 and the recess 14, which are shaped and have dimensions as mentioned above, against the top end of the cylindrical portion 10, the top end of the cylindrical portion 10 is forcibly expanded outward in the diameter directions for caulking, to thereby form the caulking portion 9. The inner ring 3 is firmly held between the caulking portion 9 and a step surface 23 of the stepped part 8 formed at the inner end of the hub 2, whereby the inner ring 3 is fastened to the hub 2.
A first outer raceway 15 and a second outer raceway 17 are formed in the inner surface of the outer ring 4. The first outer raceway 15 is formed in the outer peripheral surface of the middle portion of the hub 2, while being confronted with the first inner raceway 7. The second outer raceway 17 is formed while being confronted with a second inner raceway 16 formed in the outer peripheral surface of the inner ring 3. A plurality of rolling elements 5, while being rollably held within an element holder 18, are located between the first inner raceway 7 and the first outer raceway 15. A plurality of rolling elements 5, while being rollably held within an element holder 18, are located between the second inner raceway 16 and the element holder 18. In the illustrated instance, the rolling elements 5 are balls; however, in the case of the wheel-support rolling bearing unit for motor vehicles which is heavy, those may be tapered rollers.
To assemble the wheel-support rolling bearing unit 1 into the motor vehicle, the outer ring 4 is fixed to a suspension device with a second flange 19 formed on the outer peripheral surface of the outer ring 4, and the wheel is fixed to the first flange 6. As a result, the wheel is rotatably supported on the suspension device.
For a work to form the caulking portion 9 by plastically deforming (expanding for caulking) the cylindrical portion 10 in order to form the wheel-support rolling bearing unit 1 thus constructed and operated, it is preferable to use a swing press 20 as shown in FIG. 7. The swing press 20 is made up of a force piston 12, a holding tool 21 and a holder 22. In forming the caulking portion 9 by expanding for caulking the cylindrical portion 10, the force piston 12 is displaced in a swing manner while pushing upward the hub 2 with the aid of the holder 22. In a state that the center axes of the force piston 12 and the hub 2 are inclined at an angle θ with respect to each other, the force piston 12 is turned about the center axis of the hub 2, while being in contact with each other. In forming the caulking portion 9 by the swing press as just-mentioned, a part of the circumference of the force piston 12 is pressed against the cylindrical portion 10, so that the caulking expanding work of the caulking portion 9 continuously progresses in part in the circumferential direction. For this reason, in forming the caulking portion 9 by ordinary forging process, a load acting on the cylindrical portion 10 may be reduced during the forging process. The holding tool 21 prevents the inner ring 3 and the hub 2 from moving in the radial direction during the caulking expanding work of the caulking portion 9 by the force piston 12.
A technique that in the above structure, a portion of the outer peripheral surface of the hub 2, cross hatched in FIG. 4, is quenched to be hardened to improve the durability thereof, is also disclosed in the Japanese Patent Unexamined Publication No. Hei.11-129703. Specifically, the first inner raceway 7 portion, the base end portion of the first flange 6, and the half of the base end portion of the stepped part 8 are quenched to increase hardness of those portions to about Hv550 to 900. On the other hand, the hardness of the cylindrical portion 10 which will form the caulking portion 9 is decreased to about Hv200 to 300 so that the cylindrical portion is easy to be plastically deformed.
Of those cross hatched portions to be quenched, the first inner raceway 7 portion receives a great surface pressure upon contact of it with the rolling surfaces of the rolling elements 5. Accordingly, it is hardened order to secure a rolling fatigue lifetime. The base end portion of the first flange 6 is hardened in order to prevent the base end portion from being deformed independently of a moment load receiving from the first flange 6 to which the wheel is fastened. The half of the base end portion of the stepped part 8 is hardened in order to prevent the outer peripheral surface of the stepped part 8 from being deformed independently of a fitting pressure of the inner ring 3 and a radial load that the inner ring 3 receives from the plurality of rolling elements 5, or to prevent the fretting from occurring on the outer peripheral surface of the stepped part 8 where it receives the inner ring 3 fit thereto. The step surface 23 portion of the stepped part 8 is hardened in order to prevent the step surface 23 from being deformed independently of an axial directional load acting on the inner ring 3 by caulking work to be described later, to prevent the fretting from occurring on the step surface 23 as a contact surface where it comes in contact with the outer end surface of the inner ring 3, and to prevent a corner R as a continuous portion where the outer peripheral surface of the stepped part 8 continues to the step surface 23 from being deformed as the result of the stress concentrated thereto.
In a Japanese Patent Unexamined Publication No. Hei.10-95203, as shown in FIG. 8, the outside diameter of a portion of the inner end of the hub 2a, which is protruded beyond the fitting portion of the inner ring 3, is somewhat smaller than the outside diameter of the fitting portion. Specifically, a stepped part 25 having a height H of about 0.02 to 1 mm is formed on the outer peripheral surface of the base end of the cylindrical portion 10a formed on the inner end of the hub 2a at a location closer to the second inner raceway 16 than a slanted surface 24, which is formed at the inner end opening of the inner ring 3. The slanted surface 24 is held down by expanding radially outwardly the diameter-reduced portion of the cylindrical portion 10a for caulking. When the cylindrical portion 10a is radially outwardly expanded for caulking, the bending of it starts at the stepped part 25. With provision of the expanding work, excessive force hardly applies to the cylindrical portion 10a during the caulking expanding work. As a result, the expanded portion for caulking is little damaged, for example, cracked.
The above-mentioned structure will possibly realize a small and light-weight wheel-support rolling bearing unit 1 at low cost. However, to secure the sufficient durability, reliability and cost reduction of the resultant product, it is necessary to increase the production yield by making the caulking portion 9 for fixing the inner ring 3 to the hub 2 (2a) free from damage, e.g., cracking.